This proposal examines mechanisms of acquired tolerance to protein antigens with a focus on the antigen presenting cell. The experiments will test the hypothesis that small B cells present antigen to primary helper T cells and turn them off instead of on, because small B cells lack certain accessory signals required by a potentially responsive T cell. This hypothesis offers a satisfactory explanation for the phenomenon of low zone tolerance, in which antigen is introduced in a form which is presented only by antigen-specific small B lymphocytes, bypassing other kinds of APC that are required for a positive response by primary T cells. It may also explain high zone tolerance, since at high antigen concentrations, small B cells can process and present antigens nonspecifically, and they outnumber other class II positive cells. Small B cells may play a role in self tolerance to soluble proteins including immunoglobulins and in transplantation tolerance induced and maintained by allogeneic lymphocytes. Acquired tolerance has obvious potential applications in transplantation, the treatment and prevention of autoimmune disease and chronic infections, and the effective use of the novel therapeutic proteins that can be produced by recombinant DNA techniques. To test the hypothesis: 1. small B cells will be pulsed with antigen in vivo using soluble antigen targeted specifically to membrane IgD, and 2. small B cells from a transgenic mouse expressing a foreign protein antigen will be transfused into syngeneic, non-transgenic mice. In each case, mice will be challenged with antigen in adjuvant and various CD4+ T cell responses will be measured. 3. Class II MHC restriction of helper T cell recognition will be exploited to study the effects of SCID mice (which lack functional B and T lymphocytes) reconstituted with F1 T cells and B cells of a different MHC haplotype from the SCID host. 4. The mechanism of inactivation of CD4+ T cells by B cells actin as APC will be studied in vivo and in vitro using T cells from transgenic mice in which virtually all of the T cells are specific for a particular protein antigen.